In the current pictorial information era, pictorial displaying equipments as media for transmitting information have been used. Particularly, in response to the demands for personification of displaying equipments, which act as interfaces between human beings and computers, and for space economization, a variety of flat screens or flat-panel displays including LCD, plasma display panel (PDP), electroluminescence (EL) and the like have been developed to replace the conventional displays and, particularly, relatively large and voluminous cathode ray tubes. The advancement of the technology for liquid crystal displays with characteristics of thinness, low voltage drive and low power consumption has attracted the most intense interest and, as a result, the color picture quality of some liquid crystal displays has been improved so as to satisfactorily compare with some forms of cathode ray tubes.
Generally, a liquid crystal display includes a liquid crystal layer disposed between glass plates having transparent electrodes. The glass plates also have polarization films on their front and back surfaces. The polarization films used for this kind of liquid crystal display are typically made by adsorbing iodine or special dyestuff onto polyvinyl alcohol films and then stretching the films in a certain direction. Such polarization films have a low mechanical strength in the direction of the transmission axis and tend to have a lower polarizing performance because of shrinkage by heat or moisture and, therefore, cannot be put to practical use as a polarizer.
One solution to this problem is a structure wherein cellulose acetate films as supports are fixed by interposed binder layers.
Moreover, because a polarization film based on polyvinyl alcohol produces a linear polarized light by absorbing the light oscillating only in one direction and transmitting the light oscillating in the other direction, the theoretical efficiency of such a polarization film can not exceed 50%. This is the biggest factor lowering the efficiency of LCDs. In addition, as polyvinyl alcohol is a water-soluble polymer, it has poor resistance to water and heat and can deteriorate the durability of the polarization film. As an effective means for improving water- and heat-resistance, there is employed a formalization and heat-treating process in which a cross-linked reaction with boric acid or glyoxal takes place to reduce the OH radicals.
The polarization film of cholesteric liquid crystals can improve the drawbacks of the conventional polarization film. The cholesteric liquid crystals have a selectively reflecting characteristic by which they reflect only the circular polarized light, the traveling direction of which coincides with the twisted direction of the spiral structure of the cholesteric liquid crystals and the wavelength of which is identical to the spiral pitch of the liquid crystals. The advantage of this selectively reflecting character can be used to manufacture a polarization film that can convert an unpolarized light with a certain wavelength band into a specific circular polarized light. This is to say, when an unpolarized light, which includes the right circular polarized light and the left circular polarized light at equal proportions, is projected into a cholesteric liquid crystal film having a levorotatory or dextrorotatory structure, the circular polarized light in agreement with the spiral direction of the crystal structure is reflected and the circular polarized light in the opposite direction is transmitted. The transmitted circular polarized light is converted into a linear polarized light by passing it through a quarter wave (.lambda./4) retardation film. On the other hand, the reflected circular polarized light changes its polarization direction through re-reflection on a reflector plate and then is projected back into the liquid crystal film.
Such a method of using cholesteric liquid crystal film as a polarization film could increase the brightness by two times that of a conventional absorption type polarization film capable of absorbing only 50% of the light because of the theoretical absence of light loss.
The backlight used for LCDs, however, mainly emits the light having the visible band (400-800 nm) representing the chromatic range. Therefore, when only one kind of cholesteric liquid crystal monomer is employed, the single fixed spiral pitch of the cholesteric liquid crystal limits the polarizing ability of the light having the wavelength corresponding to the spiral pitch of the liquid crystal, with the light having the remaining wavelengths transmitting through the film as unpolarized light, thereby degrading the picture quality.
A need, therefore, exists for an improved method of manufacturing a polarization film having a high polarizing efficiency in a simple process, as well as a novel raw material for the polarization film.